The present invention relates to a system for remotely identifying an object such as that disclosed and claimed in the application of Stanley R. Sternberg and John W. Lennington entitled "Remote Identification System", Ser. No. 603,927, filed Aug. 12, 1975, now issued as U.S. Pat. No. 4,025,791 as well as in the applcation of Stephen S. Wilson entitled "Object Identification System", Ser. No. 709,237, filed July 27, 1976, now issued as U.S. Pat. No. 4,121,102. The remote identifying system of this invention is adapted to identify any object which may carry a transponder of the type disclosed herein in a position so that the transponder is in optical communication with an interrogator. One particularly useful application of the remote identifying system of this invention is to identify automobiles as they enter and depart a parking structure or the like so that their entry and departure may be recorded and their identification checked against a compilation of authorized vehicles. Advantageously, the system of the present invention eliminates the necessity of stopping the vehicle to obtain a ticket or use another authorizing device such as a machine readable card. Another pertinent application involves the use of system on toll roads, where the system could eliminate toll booths and provide an automatic billing system while eliminating the traffic obstruction of the toll booths.
In the past, many systems have been suggested for remotely identifying objects. Generally, the systems can be classified into two general classes, (1) passive device systems, and (2) active device systems. The passive device systems may use labels or other structures on the object which may be read or which may cooperatively function with an interrogator to yield an identifying code. Generally, devices for reading labels such as pattern recognition systems are costly and complex, and as a result, do not lend themselves to many applications in which object identification without human assistance would be desirable. Passive device readers such as those using structures which are selectively resonant with ultrasonic vibrations or high frequency electromagnetic waves have the disadvantages of requiring an inventory of a large number of unique passive structures and very precise manufacture of the passive structures. There are further limitations relative to the number of unique codes which can be stored or transferred economically in the available time.
Active device object identification systems may be classified in the following categories: (1) high frequency or radio frequency electromagnetic communicators, (2) light beam communicators, and (3) electromagnetic field communicators. The radio frequency devices have the disadvantages of requiring compliance with Federal Communications Systems' rules and the further disadvantage of susceptibility to radio frequency interference. This susceptibility requires the use of highly directional receiving and transmitting antennae. The magnetic field devices are difficult to implement due to the shielding of the steel bodies of automobiles and the existence of strong time-varying, interfering magnetic fields from generators and other devices which are associated with the automobile.
The object identification system of the present invention avoids many of the difficulties of the prior art identification systems through the use of unique optical and electronic structures and methods. The transponder of the object identification system of this invention can operate on its self-contained power source for a period well in excess of one year without replenishment, requires no attention from the carrier of the transponder to emit its identifying code, is physically small, e.g., approximately the size of a cigarette pack, so that it may be carried by a large variety of small objects, and is capable of transmitting information rapidly enough so that, for example, a 64-bit binary number can be received several times by an interrogator at a fixed location as the object passes the location of the interrogator at speeds in excess of 60 miles per hour, thusly providing redundant interrogation even at high transit speed.
As previously indicated, the remote identifying system of this invention is especially useful as a means for identifying vehicles as they enter or exit a parking facility or as they pass under interrogators on toll roads. Presently, a number of different methods are in commercial use for identifying vehicles as they enter a parking facility. One such system is to provide each authorized vehicle with a sticker or other label which may be viewed by an attendant at the entrance of the parking facility. This method for identifying vehicles has the advantage that the driver is not required to stop the vehicle as it enters the parking facility but has the decided disadvantage of requiring the employ and constant attention of a parking attendant. The system is subject to human error and attendant inattention or dishonesty so that unauthorized vehicles or vehicles with expired leases may be allowed to enter. Another commercial system employs magnetic cards which the driver is required to insert into a slot in a card receptor at the entrance to the parking facility. The driver must stop his vehicle, thereby slowing the rate of entry of vehicles into the parking facility and inconveniencing the driver. The slow entry rate complicates the provision for traffic at the entry to the parking facility and often necessitates additional parking facility entrances to accommodate the slow entry rate. Accordingly, the cost of the parking facility is increased.
The use of such a system on toll roads is just now being seriously considered, and various systems for this application are now under test. Basically, the system must remotely identify vehicles passing through traffic lanes at highway speeds. The number storage capacity of the transponder must be very high (greater than 1 billion numbers) and the battery life in the transponder must be longer than one year. The transponder must obviously respond and transmit its data very quickly.
The object identifying system of the present invention, when used to identify vehicles entering a parking facility, has the substantial advantage of providing rapid and accurate identification of vehicles without requiring the employ of an attendant at each entrance or the stopping of each vehicle at the entrance. Accordingly, the vehicle may enter the parking facility at any practical speed. Furthermore, this system provides a high degree of user convenience. In addition, the digital code used by the system of this invention permits automatic parking control, computer accounting, billing, audit and vehicle inventory. For example, in the preferred embodiment, the digital code representing the vehicle entering the parking structure is provided to a controller for automatic comparison to a compilation in the memory thereof of authorized vehicles, time recording, and vehicle entry and departure recording so as to provide accurate and highly reliable control of the access of the parking facility vehicle inventory and accounting.
In its preferred form, the object identifying system according to the present invention includes a transponder which is carried by the object to be identified and an interrogator which may be in a fixed position and is in optical communication with the transponder. Preferably, the transponder and the interrogator are capable of detecting and emitting light pulses, e.g. pulses in the infrared band. The transponder is provided with a memory which stores a digital code which uniquely identifies the object carrying the transponder. In the preferred embodiment, the memory of the transponder is a recirculating shift register which is programmed with the digital code. With minor modification of the transponder circuitry, a random-access memory could be used as the storage medium.
While the present invention shares some common features with the systems disclosed in the listed patents, the principal distinction between these prior art systems and the present invention is that the system of the present invention uses nonsynchronous communication between transponder and interrogator rather than synchronous communication. In synchronous systems, the transponders transmit their data bits in synchronism with light pulses received from the interrogator. In other words, the interrogator receives or interprets a single bit of information from the transponder in response to each light pulse which is transmitted by the interrogator. Also, the time between the interrogator emitted pulse and the received or implied data pulse is relatively well-known. In a non-synchronous system, the interrogator light pulse which is repeated at a rate of 60-120 pulses per second is merely used to activate the transponder and shift it from the quiescent state into the transmitting state, and no special temporal relation exists between interrogator light pulses and transmitted data bits. In other words, the light pulse from the interrogator simply serves to turn on an optically-activated switch within the transponder which then remains in the active (or transmitting) state for a predetermined time which is deemed to be sufficient for the transponder to accurately transmit its identifying code to the interrogator. This switch will respond to any single light pulse emitted by the interrogator.
Note that in applications where a manually-activated transponder could be used, the optically-activated switch in the transponder can be replaced directly with a manual switch. Such a direct adaptation of the transponder in a synchronous system is not possible.
The development of the non-synchronous system of the present invention was accomplished in response to user requirements which are not adequately served by a synchronous system. Primary among these are system cost and performance. The following is a comparative listing of some important performance/cost characteristics for the synchronous and the non-synchronous systems.
1. Interrogator transmitter
(a) Synchronous system PA1 Banks of expensive high-powered LED's or equivalent light sources are required to produce CW light pulses from the interrogator which can be detected by the transponder. The LED's and their associated driver circuits generate a great deal of heat, which must be dissipated by large and expensive heat sinks. No adequate replacement has been found for the LED's as a CW light source. PA1 (b) Non-synchronous system PA1 The preferred light source is a xenon flashtube, which generates light pulses of less than 10 usec. duration at a repetition rate of 60-120 pulses per second. The flashtube and driver circuitry have a substantial cost advantage over that associated with the LED's and drivers. The power input to the flashtube is only about 35 watts, most of which is converted to light output so there is negligible heat generated by the flashtube and its circuitry. PA1 (a) Synchronous system PA1 The transponder receiver must contain high-gain, linear, high slew rate, tuned amplifiers to receive the light pulses from the interrogator and reject unwanted signals. These amplifiers are expensive, contain many parts, and are somewhat tempermental. Also, they consume considerable quiescent current from the transponder battery. PA1 The transponder receiver is composed entirely of switching circuitry and, as such, its design is much less critical than the design of linear tuned circuitry. Also, since the non-synchronous transponder utilizes swithing circuitry which only has to turn on rapidly (it can turn off relatively slowly), the switching devices can be in, or nearly in, the off state when not activated. Thus, if the receiver circuit is properly designed, the battery current consumption of the nonsynchronous transponder receiver can be less than one-tenth that of its synchronous counterpart. PA1 (a) Synchronous system PA1 The rate at which data can be transmitted is tied directly to the rate at which pulses can be received and accurately resolved. Since amplifier bandwidth is a direct function of amplifier current consumption, the data rate is ultimately a function of current consumption. At acceptable current consumption levels, the synchronous transponder data rate has been limited to less than 7000 bits/second. Also, since the amplifiers are tuned, the receiver has a rather long transient response which is manifested as a delay of several milliseconds between the time when light pulses impinge upon the transponder and the time when the transponder begins to emit data. PA1 (b) Non-synchronous system PA1 Since the transponder transmitting capability is dependent upon the receiver section only to the extent that it must be turned on by the receiver section, it can transmit data at a rate in excess of 100,000 bits/second. A data rate of 50,000 bits/second was arbitrarily chosen for the preferred embodiment described herein, and could easily be increased if desired. PA1 (a) Synchronous system PA1 The usable number of distinct numbers which can be stored in a transponder depends upon the optical configuration of the interrogator receiver and transmitter, the data rate of the transponder, the speed at which the transponder moves and the level of data redundancy which is required to maintain an acceptably low error rate. Systems using all of the variables above to best advantage have a usable transponder number capacity of 8000 numbers for a maximum expected transponder speed of 30 miles per hour, and considerably less than that for a maximum transponder speed of 60 miles per hour. PA1 (b) Non-synchronous system PA1 The non-synchronous transponder uses a 30-bit binary code which provides over one billion distinct numbers. The system of this invention in its preferred form can reliably read one of these numbers from a transponder moving in excess of 120 miles per hour. This level of performance can be further improved through modification of the interrogator optical activation mechanism. PA1 (a) Synchronous system PA1 Since all high-power LED's emit at roughly the same wavelength and since the synchronous system uses these LED's for communication in both directions at a synchronized pulse rate, there is a problem with interference between the two signals. PA1 (b) Non-synchronous system PA1 Since the flashtube pulses only at a rate of somewhere between 60 and 120 pulses per second and since the width of these pulses is only a few microseconds, interference occurs only briefly and relatively infrequently. In actual operations, interferences prove to be irrelevant to proper system operation. PA1 It is estimated that at current prices a non-synchronous system fulfilling a given overall system requirement can be built for less than one-half of the cost of a synchronous system which fulfills the same requirement.
2. Transponder receiver
(b) Non-synchronous system
3. Data transmission rate of transponder
4. Usuable transponder number capacity
This number capacity, while adequate for parking garage use, is totally inadequate for toll road use.
5. Transponder--interrogator optical interference
6. System cost